Process of etching



J. M. BOOE Sept. 1, 1936.

PROCESS OF ETCHING Filed Dec. 31, 1934 MHGAIFIED msw XNVENTOR Jrmmv M 0 M/YG/VlF/ED VIEW ATTORN EY heretofore used.

Patented Sept. 1, 1936 UNITED STATES PATENT OFFICE PROCESS OF ETCHIN G ware Application December 31, 1934, Serial No. 759,898

' 16 Claim.

This invention relates to methods of etching and the product thereof and more particularly to the preparation of etched surface metal electrodes.

An object of the invention is to produce an etched surface of improved characteristics.

Another object is to provide a galvanic etching process for producing improved etched surface electrodes and other articles.

A further object is to produce etched plates suitable for use in electrolytic cells, condensers, asymmetric cells, lightning arrestors and the like.

A still further object is to increase the effective electrode surface area of a metal plate, such as a plate of aluminum foil.

Another object is the provision of an etching process which is more economical than those Other objects will be apparent from the following disclosure and the accompanying drawing together with the appended claims.

The invention may be said to involve the use of a combination mechanical, chemical and electro-chemical etching process. The metal sheet or electrode to be etched is placed in juxtaposition with an auxiliary and cooperating metal electrode which it preferably contacts at a plurality of points or small areas distributed over its surface. Both electrodes are immersed in an etching electrolyte which has the property of attacking and etching the metal sheet but which does not substantially attack the auxiliary electrode. By virtue of the relationship of the electrodes and electrolyte, a galvanic action occurs which accelerates the electrolyte attack on the metal sheet 'to etched.

The invention accordingly comprises the methods of manufacture, features of construction, combination of elements and arrangement of parts referred to above or which will be further brought out and exemplified in the disclosure hereinafter set forth, including the illustrations in the drawing, the scope of the invention being indicated in the appended claims.

For a fuller understanding of the nature and objects of the invention as well as for specific fulfillment thereof, reference should be had to the following detailed description taken in connection with the accompanying drawing, in which:

Figure l is a diagrammatic illustration, in sectional elevation, of an apparatus for carrying out the etching process;

Figure 2 is an enlarged sectional view of a meapparatus during the etching operation;

Figure 3 is a view, in similar section to Figure 2, of parts arranged according to a modified form of the invention;

Figure 4 shows a sheet etched without the use of a screen; and. 5

Figure 5 shows a sheet etched according to the present invention.

Like reference characters indicate like parts in the several figures of'the drawing.

While a preferred embodiment of the invention will be described herein, it is contemplated that considerable variations may be made in the method of procedure and the construction of .parts without departing from the spirit of the invention.

Figure 1 of the drawing shows apparatus designed for the continuous etching of metal sheet or foil according to one form of the invention. It is understood, however, that the invention also contemplates other forms and modifications of etching methods and apparatus adapted for continuous or for batch operations carried out either manually or automatically. The method used in any particular case will, of course, depend upon the composition and dimensions of the plates to r be etched, the quantity to be etched at any time and the characteristics or" the etching bath.

While the invention is applicable to the etching of metal electrodes, sheets, plates and foils of a wide variety of types and compositions,. it is especially suitable for the etching of metal sheets composed of film-forming metals, i. e., metals which form an adherent current blocking or asyimnetrically conductive film when subjected to electric current in the presence of a suitable electrolyte. Such metals find application in devices such as electrolytic cells, condensers, rectihers and the like. Among the metals of this type are to be included aluminum, tantalum, magnesium, etc. The etching substantially increases the effective electrode area and hence affords a greater dielectric capacity.

The etching apparatus illustrated in Figure l, for etching a metal sheet or foil 9, comprises a, tank l0 containing an etching bath H of suitable strength and composition for producing the desired depth of galvanic etch upon the foil 9. For aluminum foil, a bath of hydrochloric acid is found to be satisfactory. Other etching baths which may be satisfactorily used for aluminum include sulphuric acid, hydrofluoric acid and sodium hydroxide solution. Preferably, the electrolyte used should be of a composition such that its anion forms soluble compounds with the metal being etched.

against the face of screen it on the periphery of drum I 2 for the one-half or more of the periphery of drum 52 which is below the surface of the bath. Roller l5 may be arranged to apply a constant tension to screen it, as shown.

A second tank I9 is situatedat the left of tank l9 and is filled with water which may, if desired,

be boiling distilled water, for washing the plates or foils after etching. Suitable rollers 29 are arranged to lead the foil from the etching bath through the washing tank. A spindle '23 is pro= vided with a crank 2% for drawing the foil through the apparatus and forming a roll thereof.

The foil 9 to be etched is arranged to pass through the etching bath with its surfaces held in close contact with screens l3 and it. For this purpose after passing through cleaning and rinsing solutions the end of foil 9 is led through the bath between the screens 93 and it, where both are held together along the lower part of the periphery of drum l2. The relationship of the foil and screens will then be as illustrated in the fragmentary section shown in Figure 2.

Screens 53 and it are preferably of rather fine texture, screens having or meshes to the linear inch, for example, being suitable. The metal forming the screen is so chosen as to pro mote galvanic etching of the foils or-plates when they are placed in contact with the screens in the etching solution. For etching aluminum foil screens of copper or silver are found to be especially applicable.

In general, screens formed of the metals be= low hydrogen in the electrochemical series of metals will be found most suitable since they are not subject to attack by the acid or etching bath. These metals include antimony, bismuth, arsenic, copper, silver, palladium, platinum, gold, etc.

Some metals above hydrogen in the electrochemical series may be used for the screen, however, without being unduly attacked by the etch= ing bath since they will be protected from solution by the cathodic charge imparted to the screen when in contact with the foil of H. mm (or other metal high in the electrochemical series). Thus nickel or cadmium screens may be used.

The metals high in the electrochemical series may be said to be relatively electronegative and these low in the series relatively electropositive, these terms having the meanings now generally adopted in the electrochemical art. See Transactions of the American Electrochemical Society, Volume 36, pages 3-15, -l9la."; "us copper is electropositive to aluminum.

In general, it has been foundthat the most satisfactory characteristics of etch have been obtained where the galvanic couple formed by the plate or foil to be etched, the screen and the acid bath generates a potential difference in the neighborhood of .45 or .5 volt.

It is of considerable advantage to press or roll the etching screen before use in order to obtain the greatest number of contacting points, it being readily appreciated that in a woven wire screen, there is considerable irregularity in the height of the raised points. Pressing or re brings substantially all of the raised portions of the weave to the same plane.

In some cases, it may be found desirable to electro-plate the etching screens. In this way, a worn screen'may be built up with-the metal from which it is composed. Likewise, a base metal screen, for example, copper, may be plated with amore valuable or mechanically weaker metal, such as gold or silver.

The etching process may be carried out as follows: The foil 9 to be etched may first be cleaned by washing in a 5% solution of caustic soda or a solution of hydrofluoric acid and rinsing. The

foil while still wet, is led through the etching bath El, as shown in Figure 1, between screens i3 and it, then through the washing tank I!) which may contain boiling distilled water and finally rolled up on spindle 23. The foil 9 may be drawn continuously through the bath H by turning crank 26 the etching taking place with 20 the foil and screens in close contact.

It is of considerable importance that the foil be prevented from becoming dry after it has been cleaned in the caustic or cleaning acid solution and rinsed. The foil should preferably enter the 26 etching solution while still wet from the rinse water. It has been found that if the foil dries, even momentarily, between cleaning and etching very little etching can be obtained. This is believed to be due to the formation of a superso ficial oxide film on the aluminum orother metal of the foil, which prevents contact between the metal screen'and the metal of the foil. The speed of etching may be regulated to obtain a desired depth of etch. With aluminum' foil, copper gauze and 25% hydrochloric acid solution, an etching time of 40 seconds has been successfully employed at a temperature of 40 N. The type and rate of etching may also be varied by controlling the temperature of the 1'1 bath, a temperature above room temperature generally being preferred. By 25% hydrochloric acid is meant a solution of 1 part hydrochloric acid and 3 parts of water by volume, that is, a solution containing appro lately 10g 5 HCl per liter. According to a modified form of the invention porous layers 25 and it (Figure 3) are provided as coverings for the non-contacting surfaces of the screens. These layers may be formed of fabric, such as cotton gauze or canvas, or of sponge rubber or perforated semi-hard rubber. They tend to inhibit the circulation of etching solution through the screens during etchingand thereby control the rate of entrance of fresh 55* electrolyte into the sphere of action. 'I'heyalso help to bind the metal gauze tightly to the foil surface to insure good contact of each segment of wire with the foil. Furthermore, the'tempera ture at the contacting surfaces will be appredill The etched foils produced according to the present invention will have greatly increased effective surface area when used for electrolytic cells, electrolytic condensers, rectifiers and the 65 like due to the variations in depth of etch caused by the presence of the contacting screen. The etched surfaces of the foils will be somewhat spongy in appearance when examined under the microscope and will have a characteristic 7o fretted or wame" appearance, as shown at 28 in Figure 5, corresponding to the weave of the screen as contrasted with the irregular, hit-or-miss ap pearance characterized by absence of any regular pattern of sheets etched by chemicals or acids to without the use of screens as shown at 21 in Figure 4.

' The advantages obtained by the present etching process appear. to be due to a combination of .5 effects and are not believed to be due entirely to any one effect.

It is believed that the advantages result, in part, from the fact that portions of the foil surfaces are protected during etching by the 10 raised parts of the weave of the contacting screens so that etching is prevented or greatly inhibited in small areas distributed uniformly over the surface of the foil. Thus the foil will not be eaten away as uniformly as it would if fully ex- 16 posed to the bath over its entire surface, and consequently the effective surface area is greatly increased.

The contact between the foil and metal screen completes a galvanic circuit, greatly enhancing 20 the rate of etching. On the other hand, the

metal screen (and the additional porous protective layer, when used,) controls the rate of flow of electrolyte to and from the zone of activity. i

A very practical combination for etching aluss minum foil appears to be 150 mesh copper gauze used in conjunction with dilute hydrochloric acid (1 part acid to 3 parts water, by volume) at room temperature or above, for example at 40 0.

Dry electrolytic condensers made from foils '30- etched according to the present invention, show markedly higher capacities than condensers of identical dimensions made from unetched foils. A sample condenser made from etched foils had a capacity of 8.78 microfarads whereas a con- 36' denser made in identical dimensions of unetched foil had a capacity of only 2.55 microfarads.

Wet electrolytic condensers having film forming surfaces etched in accordance with my process, show a similar increase in capacity.

In some cases it may be preferable, in order to improve the operating characteristics and to obtain a lower power factor, to subject the etched foil to a heat treatment. The etched foil may be heated in air, other gases or in liquids, such as 45 high-boiling alcohols and other high-boiling liquids which are chemically inert to the foil. The heat treatment has a very noticeable eifect in increasing the effective capacity and greatly decreasing the series resistance of units made from 5 such treated foil. It is believed that the beneflcial effects of the heat treatment are largely due to the removal of gases, principally hydrogen, dissolved in the metal or entrapped in the foil surface.

5 The present invention is not intended to be construed to cover etching methods in-which a metal element is not used in contact with the foil or plate being etched.

It has been proposed, in the prior art, to clean 0 and roughen plates of film-forming metals by successively immersing them in different acid solutions. Such changes as may have been produced by that method, however, were merely the results of a chemical cleaning and dissolving op- 65 eration. The present invention, on the other hand, contemplates a galvanic etch in which the metal being etched is placed in direct contact with another metal to form a galvanic circuit. In addition, the invention contemplates means 70 for covering or making contact with the metal sheet in a plurality of small areas which may be uniformly distributed over the surface whereby a novel form of surface for electrodes for electrolytic cells is produced, having greater effective 75 surface area. By the present method, I obtain a more definite and effective control of the etching solution than is possible with processes of the prior art and the surfaces of the electrodes when made in accordance with the present invention, present a fairly definite pattern as contrasted 5 with the haphazard structural appearance of electrodes etched by other processes. Furthermore, the process is more economical than those heretofore employed as less acid is used and there is a comparative absence of precipitation. 10

.While the present invention, as to its objects and advantages, has been carefully described herein as carried out in specific embodiments thereof, it is not desired to be limited thereby but it is intended to cover the invention broadly within the spirit and scope of the appended claims.

What is claimed is:

1. The method of producing etched metal electrodes which comprises providing metal elements electropositive in respect thereto in contact with surfaces of said electrodes in a plurality of small areas distributed over said surfaces and concurrently therewith exposing the assemblies thus produced to an etching bath so as to increase the effective surface area of said electrodes.

2. The method of etching metal electrodes which comprises providing forarninous metal layers electropositive in respect thereto over surfaces of said electrodes and in direct contact therewith, whereby contacted areas of said sheet surfaces are interspersed with uncontacted areas, and concurrently therewith exposing the assemblies so produced to an etching bath so as to increase the effective surface area of said electrodes.

3. The method of etching film-forming metal electrodes so as to increase the effective surface area thereof which comprises placing relatively electropositive metal screens over surfaces of said electrodes and in direct contact therewith, whereby the raised portions of the weave of said screens will rest directly upon said electrode surfaces, and exposing the assemblies so produced to an etching bath adapted to etch said filmforming metal.

4. The method of etching a film-forming metal electrode so as to increase the effective surface area thereof which comprises placing relatively electropositive metal screen against the surfaces of said electrode, said screen being formed of a metal adapted to produce a galvanic couple with the metal of said electrode, and exposing the assembly of said electrode and said screen to an electrolytic bath adapted to etch said film-forming metal.

5. The method of preparing an electrode for electrolytic condensers which comprises covering the surfaces of an aluminum body with screen formed of metal below aluminum in the electrochemical series of metals and exposing said assembled aluminum body and screen to an electrolytic bath adapted to etch said aluminum whereby the eifective capacitance area of said aluminum is increased.

6. The method of forming a plate for electrolytic cells which comprises placing a copper screen against the surface of an aluminum-sheet, exposing the assembled sheet and screen to a bath of hydrochloric acid to etch said sheet and increase the effective surface area thereof and removing the etched aluminum sheet from said bath.

'7. The method of etching a film-forming metal electrode so as to increase the effective surface area thereof which comprises placing a relatively electropositive metal element in contact with said electrode at a plurality of points dis.- tributed over the surface thereof and immersing the assembled body and element inan etching comprises 'wettingan element of film-forming metal, covering surfaces of said element with screens formed of metal below said film-forming metal in the electro-chemlcal series of metals, immersing said assembled wet element and screen in an etching bath adapted to etch said element,

and removing said element from said bath.

9. The method of etching an electrode for condensers, electrolytic cells and the like so as to increase the effective surface area thereof which comprises cleaning an element of film-forming metal in a cleaning solution, rinsing said element, covering surfaces of said element while still wet with screens formed of metal below said iilmforming metal in the electro-chemical series of metals, immersing said assembled wet element and screen in an etching bath adapted to etch said element, removing said element from said bath and washing the remaining etching liquid therefrom.

10. The method of preparing an electrode which comprises exposing to an etching bath an assembly ofthe electrode in intimate contact with a foraminous metallicv member electro-positive with respect thereto, so as to etch said electrode and increase the effective surface area thereof.

11. The method of galvanically etching an elec-' trode for electrolytic devices which comprises exposing an assembly of said electrode and a forarninous metallic member, eleotro-positive with respect thereto and in metallic contact therewith over the area to be etched, to an etching bath adapted to etch said electrode whereby the effective surface area of said electrode is increased.

12. The method of galvanically etching a metal member which comprises contacting said member at a plurality of points with an auxiliary elec trode which is electro-positive in respect to saidmember, exposing said contacting bodies to an etching bath to etch said metal member so that the surface pattern of said auxiliary electrode is substantially reproduced on the surface of said member, the eflective area of which is thereby materially increased.

' 13. The method of preparing electrodes for electrolytic condensers and the like which comprises exposing a surface of the electrode material to an etching bath and concurrently therewith exposing an auxiliary material to said bath and providing direct contact between said electrode surface and said auxiliary material at a plurality of points distributed over said surface,

these points of contact being interspersed with areas exposed to said etching bath, said auxiliary material being sufficiently electro-positive with respect to said electrode material to set up a galvanic etching action at said electrode surface when said assembly is exposed to said etching bath, whereby the eflective capacitance area of said electrode surface is increased.

14. The method of etching electrode metal for electrolytic devices so as to increase the effective surface area thereof which comprises exposing a surface of said metal to an etching bath in the presence of a relatively electro-positive metal element in contact with said surface and repeata edly using said relatively electro-positive metal element for similar etching operations on other surfaces of said electrode metal.

l5.-The method of etching a strip of electrode 5 metal for electrolytic devices so as to increase the effective surface area thereof which comprises passing said strip through an etching bath,

concurrently therewith contacting a surface of' said strip at a plurality of points distributed over said surface with a rotatable relatively electropositive metal screen and rotating said screen during the passage of said strip through said bath so as to bring said screen into contact with succeeding points. on said strip.

16. The method of etching a sheet ofelectrode metal for electrolytic devices so as to increase the effective surface area thereof which comprises passing said sheet through an etching bath, concurrently therewith contacting a surface of said sheet at a plurality of points distributed over said surface with metal elements relatively electro-positive with respect to said electrode metal and moving said sheet with respect to said elements to bring successively new areas of said sheet into contact with the same metal elements. 

